Acid pH in Tumors and Its Potential for Therapeutic Exploitation

Summary

Measurement of pH in tissue has shown that the microenvironment in tumors is generally more acidic than in normal tissues. Major mechanisms which lead to tumor acidity probably include the production of lactic acid and hydrolysis of ATP in hypoxic regions of tumors. Further reduction in pH may be achieved in some tumors by administration of glucose (±insulin) and by drugs such as hydralazine which modify the relative blood flow to tumors and normal tissues.

Cells have evolved mechanisms for regulating their intracellular pH. The amiloride-sensitive Na⁺/H⁺ antiport and the DIDS-sensitive Na⁺-dependent HCO₃^-/Cl^- exchanger appear to be the major mechanisms for regulating pH_i under conditions of acid loading, although additional mechanisms may contribute to acid extrusion.

Mitogen-induced initiation of proliferation in some cells is preceded by cytoplasmic alkalinization, usually triggered by stimulation of Na⁺/H⁺ exchange; proliferation of other cells can be induced without prior alkalinization. Mutant cells which lack Na⁺/H⁺ exchange activity have reduced or absent ability to generate solid tumors; a plausible explanation is the failure of such mutant cells to withstand acidic conditions that are generated during tumor growth.

Studies in tissue culture have demonstrated that the combination of hypoxia and acid pH_e is toxic to mammalian cells, whereas short exposures to either factor alone are not very toxic. This interaction may contribute to cell death and necrosis in solid tumors. Acidic pH may influence the outcome of tumor therapy. There are rather small effects of pH_e on the response of cells to ionizing radiation but acute exposure to acid pH_e causes a marked increase in response to hyperthermia; this effect is decreased in cells that are adapted to low pH_e. Acidity may have varying effects on the response of cells to conventional anticancer drugs. Ionophores such as nigericin or CCCP cause acid loading of cells in culture and are toxic only at low pH_e; this toxicity is enhanced by agents such as amiloride or DIDS which impair mechanisms involved in regulation of pH_i. It is suggested that acid conditions in tumors might allow the development of new and relatively specific types of therapy which are directed against mechanisms which regulate pH_i under acid conditions.